The extended Republican presidential primary has left many GOP donors paralyzed — unsure of whether to invest in the upcoming battle against President Barack Obama or focus on Congressional races.

Party insiders increasingly believe that former Massachusetts Gov. Mitt Romney will win the nomination, a development that would likely open the donor spigot for the general election. But a victory by former Speaker Newt Gingrich (Ga.) or ex-Sen. Rick Santorum (Pa.) would probably have the opposite effect. A GOP money machine skeptical of the party’s White House prospects would likely spend instead on House and Senate races as the best hope for a November gain.

According to interviews with a dozen mostly Washington-based Republicans, including Capitol Hill aides, fundraisers, strategists and K Street operatives, the GOP’s Congressional candidates and national campaign committees would prefer to avoid such a financial windfall, believing the party’s best chance of holding the House and flipping the Senate is to field a strong, well-funded presidential campaign that runs a sophisticated voter-turnout operation.

“This is a discussion that has been happening in all corners of the GOP consultant class,” a well-placed Republican said Monday.

There are some Republican strategists less concerned with the nominee than the fact that the GOP primary is diverting attention from Obama’s record and allowing the president to rebuild politically. They worry that a stronger Obama could help Democrats downticket.

Republicans are additionally concerned that they cannot produce a coherent, unified election-year message until their primary produces a nominee for them to rally around.

One Republican strategist focused on House races said both Romney and Santorum offer GOP Congressional candidates opportunity for success — just different ones. The strategist emphasized that the key is getting the 2012 campaign to the point where the House playing field is set and Obama is the focus. “Until we get there, I don’t care much about the other stuff,” this individual said.

But in comparison to Gingrich, Santorum and Rep. Ron Paul (Texas), the consensus among GOP operatives is that Romney provides Republicans with their best chance to hold on to control of the House and take a majority in the Senate. These Republicans concede that Romney doesn’t stir passion among the party faithful, but they say they fear that the other candidates might not just lose, but sink the ticket altogether.

Should Santorum or Gingrich come from behind in the race for delegates and win the nomination, Republican operatives with relationships in the House and Senate agree that the National Republican Congressional Committee and the National Republican Senatorial Committee would adjust their pitch to donors. The committees would likely contend that the presidential race is a lost cause and that Congressional campaigns constitute the only worthwhile investment of GOP money.

A GOP lobbyist predicted that the sales pitch would work. “The [Republican National Committee] would be the third choice behind the NRSC and NRCC. Super PACs would divert everything to trying to hold the House and win competitive Senate seats,” this Republican said.

Even if Romney remains on course to win the nomination and locks it up by the time of the last primary in June, there is a chance that this could happen anyway, a GOP media consultant said. “Of course this will be the case if Romney isn’t the nominee,” this consultant said. “But there is an increasing anxiety that some donors would focus their money on the House and Senate because of a feeling that a beaten, battered Romney can’t beat Obama anyway.”

This scenario has played out in the past.

In 1996, as it became clear that President Bill Clinton was on pace to defeat Sen. Bob Dole (Kan.), Republicans turned their focus to Congressional races, spending heavily to ensure the GOP held the House and the Senate. The party even ran on a message of divided government and urged voters to choose Republicans downballot to maintain a check on Clinton.

The effort worked, but Republicans say they would prefer to avoid a reprisal. “The message would be similar to the ’96 campaign of checks and balances. I have yet to encounter anyone who is willing to celebrate that prospect, however,” the well-placed Republican said.

In the interim, the protracted Republican primary is splitting the attention of major donors, particularly in states such as Florida, Indiana and Wisconsin, which have multiple competitive races up and down the ticket.

According to a GOP fundraiser, some donors are “unsure” of where to invest, given the unsettled nature of the White House contest and competing priorities at the Congressional level. House Republicans have been selling their status as the only existing check on Obama and full Democratic control of Washington, while Senate Republicans have been campaigning on a favorable playing field that finds them four seats shy of controlling the chamber.

Republican donors are giving, as evidenced by the millions of dollars raised by the various Republican third-party groups and the political action committees supporting the presidential candidates, known as super PACs. But some Republican strategists who raise third-party money for Congressional contests say the unsettled presidential picture has complicated their effort.

“Until many of these donors get closure on the nomination fight, they aren’t engaging in House and Senate races with big checks,” one such Republican said.